The broad objectives of this research are to investigate further the mechanisms whereby the Lyme disease (LD) spirochete, Borrelia burgdorferi (Bb), is perpetuated and disseminated in natural foic including modes of transmission to humans and other animals, and to identify risk factors associated with residency in an endemic area of the Far West. A human community located in a high-risk site will be followed prospectively for 2 years, and an intensive ecological study of the potential tick vectors/vertebrate hosts of Bb will be conducted simultaneously there. Risk factors will be assessed by distributing questionnaires 3 times (entry, 1-year, 2- years) to all participants to solicit their responses to various categorical variables. Serum specimens obtained periodically from each person and from their dogs and horses will be tested for antibodies to Bb by indirect immunofluorescence. Another aim will be to determine the role of birds in the ecology/epidemiology of LD in an oak-woodland community located in an endemic area of the Sierra Nevada foothills. Various avian species will be captured, inspected for ticks, bled, banded and released. Their bloods and associated ticks will be examined for spirochetes by standard microbiological procedures, and tick prevalence/abundance will be correlated with avian habitat use and foraging behaviors. Spirochetal strains isolated from arthropods or vertebrates during ecological studies will be characterized immunochemically. Population densities of western fence lizards and rodents will be estimated in 2 habitats by pitfall-trapping, and a serologic assay will be developed to determine the suitability of this lizard species as a sentinel animal for surveillance of LD. The reservoir competency of deer mice for Bb will be determined experimentally by feeding Bb-infected ticks on them and monitoring the durations of the resultant spirochetemias, capacity of these rodents to infect normal ticks, clinical signs of infection, etc. The vector competency of 4 species of ticks for Bb will be evaluated after these ticks have fed on spirochete-infected vertebrates or on liquid food substances containing spirochetes through an artificial (parafilm) membrane. The distribution of Bb in these ticks and their ability to transmit spirochetes via eggs, from stage-to- stage, and by the bite will be assessed. Lastly, the diel questing behavior of I. pacificus subadults on grass stems in relation to weather and temporal factors will be studied outdoors in tick arenas. The effects of exposure to direct sunlight or of spirochetal infection on questing by immature ticks also will be determined.